Travelling from England to Northern Ireland via Ireland

Our trip to Northern Ireland got cancelled last year due to lockdowns but we have a credit for the ferry crossing that I'm tentatively looking at rebooking.

Our journey plan is this:

Leave home in England (my partner and I are both British)
Drive through Wales
Get the ferry from Holyhead to Dublin
Drive up to Northern Ireland immediately from the ferry to stay in a holiday cottage
Stay from Thursday and leave on Monday
Drive back from NI to Dublin ferry port
Get the ferry back from Dublin to Holyhead
Drive home from Holyhead

This would be our first ever trip to NI. I can get my head around all the stuff about only meeting friends outside, etc. etc., and I know under current rules that we're allowed to travel anywhere in the UK, but what I can't get my head around is how the trip would be affected by the couple of hours in Ireland.

Edit: to address some comments, our booking credit is with Irish Ferries so unfortunately Holyhead->Dublin or Pembroke->Rosslare are our only choices of route.

The IE government website has information about entering Ireland during the pandemic: https://www.gov.ie/en/publication/b4020-travelling-to-ireland-during-the-covid-19-pandemic/

The NI government website also has information about travelling during the pandemic: https://www.nidirect.gov.uk/articles/coronavirus-covid-19-travelling-within-common-travel-area

The UK government website has this information: https://www.gov.uk/guidance/covid-19-coronavirus-restrictions-what-you-can-and-cannot-do

But despite reading all of these pages, I still can't work out:

If our trip is viable (it's not possible to do a long weekend in NI if we'd have to quarantine for 10 days in an Irish hotel beforehand!)
What paperwork / tests we have to do and when (it's not possible to take a test 2 days before entering the UK if we've only been out of the UK for 3 hours)

We wouldn't be stopping in Ireland, purely transiting through in our car. The total length of the trip would be 5 days, with only a couple of hours driving in Ireland on day 1 and day 5.

So my question is, what are the practicalities of taking a long weekend trip to Northern Ireland from England, via Wales/Ireland?

Thanks in advance.